Depending on the terrain and slope of the road it takes the average person around 2 hours to ride a bike 15 miles.
If you pedal at a faster speed then it may only take you 1 hour and 30 minutes but at an average cycling speed you can plan on at least 2 hours to ride the bike 15 miles.
If you have to climb hills then it can take you longer but if you have downhill roads then it can speed the time up you need to go the 15 miles.
That is because gravity will help you out by allowing you to coast down the hill without needing to pedal.
You might want to stop and take a break during the bike ride so that could increase the amount of time it takes you to ride your bike 15 miles.
15 miles seems like a short distance in a car but when you walk or ride a bike that same distance it seems much father.